Add poc files

commons/readme.txt

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+This directory builds a common.cma library and also optionally
+multiple commons_xxx.cma small libraries. The reason not to just build
+a single one is that some functionnalities require external libraries
+(like Berkeley DB, MPI, etc) or special version of OCaml (like for the
+backtrace support) and I don't want to penalize the user by forcing
+him to install all those libs before being able to use some of my
+common helper functions. So, common.ml and other files offer
+convenient helpers that do not require to install anything. In some
+cases I have directly included the code of those external libs when
+there are simple such as for ANSITerminal in ocamlextra/, and for
+dumper.ml I have even be further by inlining its code in common.ml so
+one can just do a open Common and have everything. Then if the user
+wants to, he can also leverage the other commons_xxx libraries by
+explicitely building them after he has installed the necessary
+external files.
+
+For many configurable things we can use some flags in ml files, 
+and have some -xxx command line argument to set them or not,
+but for other things flags are not enough as they will not remove
+the header and linker dependencies in Makefiles. A solution is
+to use cpp and pre-process many files that have such configuration
+issue. Another solution is to centralize all the cpp issue in one
+file, features.ml.in, that acts as a generic wrapper for other
+librairies and depending on the configuration actually call 
+the external library or provide a fake empty services indicating
+that the service is not present. 
+So you should have a ../configure that call cpp on features.ml.in
+to set those linking-related configuration settings.
+